Detection of weak light signals is a common requirement in many areas of science and technology. The background that prompted the invention of the MPA is in the field of radiation detection, although the MPA has applications in other fields.
In the detection of radiation, one common method involves the use of scintillators (such as NaI(TI)). Good summaries of scintillators and their properties can be found in many standard reference books on radiation detection (e.g. G. F. Knoll, Radiation Detection and Measurement, third edition (John Wiley & Sons, 2000) Chapters 8, 9 and 10). When radiation such as a gamma ray, beta particle, alpha particle or neutron impinges the scintillator, the latter emits a short flash of light. This light is usually detected by a photomultiplier tube (PMT), or more recently, by a newer photodetector technology called a microchannel plate (MCP). The function of the PMT or MCP is to convert the weak light signal into a burst of electrons that is amplified to a level needed by conventional electronics used for pulse analysis. Both PMTs and MCPs operate in a vacuum because high-sensitivity photocathode materials (which perform the conversion of light to electrons) are extremely sensitive to gases that can chemically attack or “poison” the thin photocathode layer. This is particularly true for photocathode materials that are sensitive in the visible region of the optical spectrum, which are typically alkali metal based (e.g. S-11 photocathodes).
The application of high voltage to PMTs and MCPs creates strong electric fields that accelerate and focus the photoelectrons from the photocathode to strike an adjacent surface, coated with a special material that produces high secondary electron emissions, resulting in an increase in the number of electrons. Further amplification is done by repeating the electron bombardment process. In the PMT, this electron amplification is done by a series of “dynodes” which are conductive foils separated from each other, but connected by an electric field to accelerate and focus the electron burst to the receiving dynode. In a typical PMT, 8 to 12 dynodes are used to achieve electron gains in the order of 105 to 108. The amplified signal is collected on an anode—a conductive foil or a wire—from which the amplified electronic signal exits from the vacuum, ready for conventional electronic processing. In the MCP, the amplification is done inside microscopic channels, lined with the secondary electron emissive material. The channels are commonly at an angle to the face of the MCP to reduce positive ion feedback. The MCP is generally made of glass and the microchannels are typically 5-100 μm diameter, lined with PbO. The MCPs are made by fusing tiny glass tubes to form a boule and cutting the boule to a desired MCP thickness, usually at 8°-15°. A good description of MCPs is given by J. L. Wiza, Nucl. Instr. & Meth. 162 (1979) 587-601.
Due to technical and cost issues associated with their manufacturing processes, PMTs and MCPs are relatively small. PMTs are commonly only 2″ to 3″ in diameter, although large 20″ diameter tubes have been made. Currently, MCPs are only commercially available in sizes up to approximately 3″ in diameter. The complexity of manufacturing translates into fairly high costs for these devices, currently from several hundred dollars to well over a thousand dollars each. For certain applications, where large area detectors are required, the use of PMTs or MCPs can become prohibitively expensive.
Over the last two decades, the advent and widespread use of microelectronics has led to a technological revolution in economical manufacturing of various electronic sub-components. In particular, the production of circuit boards of various designs at reasonable volumes can be done for tens of dollars. One new radiation detection technology that has taken advantage of the low cost of modern circuit board production is the Gas Electron Multiplier (GEM), now used extensively for experiments in high-energy physics. A GEM (F. Sauli, Nucl. Instr. & Meth. A386 (1997) 531-534) consists essentially of a circuit board (a non-conducting substrate with a thin Cu layer on each side of the substrate) containing a regular array of tiny channels through the board. When a voltage is applied across the two sides of the board, strong electric field lines are formed through the channels. The GEM uses such a board in a gas medium, such as the type of gas (argon-methane) used in common gas counters. When radiation interacts with the gas, electron-ion pairs are produced. The electrons are guided to the closest channel and are accelerated by the electric field in the channel, where collisions with gas molecules inside the channel produce more electron-ion pairs. Thus, the channels in a GEM serve as tiny electron amplifiers and the GEM gas provides the agent for electron multiplication. Due to the small size of the channel, GEMs provide excellent spatial resolution for imaging charged particles transversing the gas. GEMs evolved from the use of large gas counters to detect high-energy charged particles and the need to define their trajectories in order to determine their energies and particular species. Recent advances in GEM technology have led to the thick GEM (THGEM) (L. Periale, V. Peskov, P. Carlson, T. Francke, P. Pavlopoulos, P. Picchi and F. Pietropaolo, Nucl. Instr. & Meth. A478 (2002) 377-383) and RETGEM (G. Charpak, P. Benaben, P. Breuil, A. Di Mauro, P. Martinengo and V. Peskov, IEE Trans. Nucl. Sci. 55 (2008) 1657-1663). These differ from the original GEM in the use of larger channels (˜0.3 mm) and the coating of the ends of the channel with a higher resistivity material (relative to Cu) to allow for more robust operation.
An alternative current development of the GEM technology is being pursued by several groups (e.g. R. Chechik and A. Breskin, Nucl. Instr. & Meth. A595 (2008) 116-127; H. Sakurai, F. Tokanai, S. Gunji, T. Sumiyoshi, Y. Fujeta, T. Okada, H. Sugiyama, Y. Ohishi and T. Atsumi Jour. Phys. Conf. Series 65 (2007) 012020). These groups are working on the development of a gaseous photomultiplier based on GEM technology i.e. a GEM PMT. In essence, these groups are replacing the standard dynode structure of a PMT in a vacuum with a GEM assembly and its counting gas. The GEM PMT is housed inside a sealed enclosure that has a glass window not far from the board surface. The inside of the glass window (close to the board surface) is coated with a photocathode material, similar to that of a PMT. If a scintillator (e.g. NaI(TI)) is placed against the outside of the glass window, any scintillation from the radiation sensor (in the form of a weak light pulse) would pass through the glass window to impinge the photocathode. Electrons emitted by the photocathode would be drawn towards the board surface. These electrons would produce electron-ion pairs in the gas layer between the photocathode and the board. These electrons in turn would be guided into the channels of the board by the shaped electric field where further electron amplification occurs, identical to the operations of a GEM. If additional amplification is required, additional boards can be added to achieve the desired electron signal needed for conventional electronic processing. Some success with GEM PMTs has been achieved with CsI as the photocathode (A. Breskin, A. Buzutuskov, R. Chechik, B. K. Singh, A. Bondar and L. Shekhtman, Nucl. Instr. & Meth. A478 (2002) 225-229; A. V. Lyashenko, A. Breskin, R. Chechik, J. F. C. A. Veloso, J. M. F. Dos Santos, and F. D. Amaro, 2009 IOP Publishing Ltd. And SISSA, doi: 10.1088/1748-0221/4/07/PO7005) because it is not extremely reactive with contaminants in the counting gas. Unfortunately, CsI is sensitive to only UV radiation and not to visible light around 450 nm such as produced by many common scintillators. Attempts to develop gas PMTs for visible light have been met with limited success (M. Balcerzk, D. Mormann, A. Breskin, B. K. Singh, E. D. C. Freitas, R. Chechik, M. Klin and M. Rappaport, Trans. Nucl. Sci. 50 (2003) 847-854) because the reactivity of the K—Cs—Sb limits the stability of the photocathode to only a few months, despite care in avoiding contaminant poisons. There are on-going efforts to try to protect the rare-earth photocathode by covering it under ultra-thin layers of less-reactive CsI.